I honestly wouldn't worry about it too much unless she is showing other unusual signs of puberty.
My DS has had stinky armpits since about 3.5. Especially in the summer when he is outside a lot and sweating. He bathes daily. I do have some natural deoderant for him that I use on occassion, and I put baby power in his arm pits every night to help. That seems to do the trick.
I mentioned it to his daycare teacher (large center) and the teacher said you would not believe how stinky the kids are. She said it is terrible in pre-school age group. She said it is more common than not to have a kid with stinky armpits in preschool. I did mention it in passing to his pediatrician at his well visit and she said it is completely normal and not to worry.